Accelerator Mass Spectrometry (AMS) is a type of isotope ratio mass spectrometry (IRMS) developed in the 1970s to directly count individual ions of very rare isotopes. AMS is an extremely sensitive method for detecting isotopes having concentrations of parts per billion to parts per quintillion. Recently, dedicated AMS instruments have been developed for quantifying radioisotopes that are particularly suitable for pharmaceutical research, particularly 14C, at natural concentrations (Vogel, (2005). “Accelerator mass spectrometry for quantitative in vivo tracing” BioTechniques 38:S25-S29, 2005; Nelson et al., 1977 “Carbon-14: direct detection at natural concentrations” Science 198:507-508; and Bennett et al., 1977 “Radiocarbon dating using electrostatic accelerators: negative-ions provide key” Science 198:508-510). In practice, AMS traces very low doses of compounds (micrograms) using extremely low radiation (<100 nanoCurie) in animal models and human subject. Some of the common clinical applications of AMS are quantitation of drug concentration in pharmacokinetic, mass balance, absolute bioavailability, microdosing, and metabolite profiling studies.
AMS is used when extreme sensitivity is required for early stage drug development. AMS quantifies the amount of radiocarbon-labeled compound in a biological sample with attomole (10−18M) sensitivity. Traditional Liquid Scintillation Counting (LSC) methods only count the isotopes that decay during the detection period. In contrast, AMS counts every radioisotope present in the sample, whether it decays or not. Since radiocarbon is relatively stable (i.e., approximately 5,730 year half-life), there is a 106-fold increase in the sensitivity of AMS compared to LSC.
AMS instruments accelerate ions to million electron volt energies where molecular isobars are destroyed yielding ions of sufficient energy for identification by characteristic interactions with nuclear particle detectors. An AMS instrument produces a beam of C-ions by bombarding the cool cesiated surface of a graphite sample with about 5 keV Cs+ ions. The C-beam produced by the sputtering of the sample by the Cs+ beam is accelerated, focused, and mass analyzed into mass 14, and 13 amu beams. Samples are measured with 0.3% precision and accuracy, machine background levels are consistently in the low 10−16(14C/12C), and chemical background are approximately equivalent to a fraction of modern of 0.004. In addition, when 100-times-modern samples are processed, no increase in background is observed, either during sample processing or during AMS measurement. This corresponds to a dynamic range for 14C analysis of 6 orders of magnitude (Zoppi et al., “Performance Evaluation of New Accelerator Mass Spectrometer at Accium BioSciences”, Radiocarbon, 49:171-180, 2007).
An AMS instrument generates 14C/12C ratios in a given graphite sample. The graphite is produced separately from a small amount of plasma, urine, tissue homogenate or fecal blend, for example. The absolute concentration of 14C in these samples is calculated by multiplying the 14C/12C ratio by the total amount of carbon in that sample. For example, the AMS ratio (commonly DPM/g carbon) is multiplied by 0.040 g carbon/mL plasma to produce the absolute 14C concentration (DPM/mL plasma). It is, therefore, required to determine the concentration of carbon in biological samples in order to perform this calculation. There are two approaches for this:    (1) Determine the carbon concentration empirically in every sample, for example, the total carbon concentration in each sample (75-100 μL) is measured by freezing the sample over liquid nitrogen in individual Costech tin capsules (Ventura, Calif.) followed by overnight lyophilization. Each capsule is then placed inside a second tin capsule, rolled into a ball and analyzed for total carbon concentration using a Carlo-Erba carbon analyzer (Pella, “Elemental organic analysis” Am Lab 22:116-25, 1990). Alternatively, total organic carbon is determined using powerful oxidation via the combination of sodium persulphate and UV oxidation at 80° C. This approach ensures that all dissolved carbon species will be detected. Highly sensitive infra-red detectors detect extremely low concentrations of carbon with excellent reproducibility in the low parts-per-billion (ppb) range.    (2) Use a reference value for carbon concentration without measuring the actual concentration in individual samples.
The second approach is suitable in samples that have consistent carbon concentration across the population and over time. Plasma samples are amenable for this approach as there is little variation in the overall carbon concentration across the population and over time. In other samples, such as urine and tissue or fecal blends, there is considerable variation in the carbon concentration across the population and over time. The carbon concentration in these samples needs to be determined empirically for accurate calculation of the absolute 14C concentration (FIG. 1 and FIG. 2).
One of the more common applications of AMS is to support clinical studies during early phase drug development. A typical mass balance study produces over 200 plasma samples, 150 urine samples and 100 fecal samples. All of the samples have to be converted to graphite prior to AMS measurement. In addition, the urine and fecal blends have to be processed separately for quantitation of total carbon concentration. This adds significant cost to the study and prolongs the release of the analytical report.
It would be advantageous and there is a need in the art to complete these clinical studies in a more rapid and cost-effective manner. The disclosed method addresses this need in the art.